Aerogels are porous materials that are produced by polycondensation reactions known in the art as the “sol-gel process”. A common feature among aerogels is their small inter-connected pores. The aerogel chemical composition, microstructure and physical properties can be controlled at the nanometer scale due to sol-gel processing. There are three major types of aerogels—inorganic, organic and carbon aerogels. Inorganic aerogels can be obtained by supercritical drying of highly cross-linked and transparent hydrogels synthesized by polycondensation of metal alkoxides. Silica aerogels are the most well known inorganic aerogels. Organic aerogels can be synthesized by supercritical drying of the gels obtained by the sol-gel polycondensation reaction of monomers such as, for example, resorcinol with formaldehyde, in aqueous solutions. Carbon aerogels can be obtained by pyrolyzing the organic aerogels at elevated temperatures.
Aerogels, e.g., carbon aerogels (also referred to in the art as carbon foams) have been produced by various methods for a variety of applications. These prior processes are exemplified by U.S. Pat. No. 4,806,290 issued Feb. 21, 1989; U.S. Pat. No. 4,873,218 issued Oct. 10, 1989; U.S. Pat. No. 4,997,804 issued Mar. 5, 1991; U.S. Pat. No. 5,086,085 issued Feb. 4, 1992; and U.S. Pat. No. 5,252,620 issued Oct. 12, 1993. Typically, efforts have been directed to the development of carbon aerogels for use as electrodes and include all forms of carbon aerogels, monolithic, granular or microspheres. Such electrodes find use, for example, in energy storage devices, e.g., capacitors and batteries, as well as for fuel cells, e.g., proton exchange membrane (“PEM”) fuel cells and electrocapacitive deionization devices, etc. These efforts are exemplified by U.S. Pat. No. 5,260,855 issued Nov. 9, 1993; U.S. Pat. No. 5,529,971 issued Jun. 25, 1996; U.S. Pat. No. 5,420,168 issued May 20, 1995; U.S. Pat. No. 5,508,341 issued Apr. 16, 1996; and U.S. Pat. No. 6,010,798, issued Jan. 4, 2000.
Additives can be incorporated into aerogels to make aerogel compositions (also referred to herein as “aerogel composites”). The role of the additives is to enhance the properties of pure aerogels or to impart additional desirable properties depending on the application. In general, aerogel composites are typically prepared using two different methods. The first one involves adding the additive to the sol prior to polymerization and the second method involves contacting the produced aerogel with a liquid or gaseous stream containing the additive.
Ye et al., Can. J. Chem. 75:1666-1673 (1997) disclose the preparation of polyacrylonitrile/platinum aerogel composites by dipping carbonized polyacrylonitrile (“PAN”) aerogels in hexachloroplatinic (H2PtCl6) solution. The precursor (H2PtCl6) was added prior to the gelatin stage. It is disclosed that incorporating the platinum precursor before the gelatin stage resulted in a more homogeneous distribution of platinum.
Pajonk et al., Preparation of Catalysts VII, 1998, 167 (1997), disclose a method to make carbon aerogels and load platinum onto the aerogels, whereby resorcinol-formaldehyde (“RF”) aerogels were obtained by polymerization in acetone instead of water and perchloric acid was used as the catalyst. After curing and supercritical extraction of acetone, the samples were pyrolyzed. Subsequently, the samples were impregnated with H2PtCl6 in acetone. Then, acetone was supercritically extracted and the sample was calcined and reduced with hydrogen. The dispersion of platinum was reported to be 23% and the platinum content was reported to be 0.44 wt %.
U.S. Pat. No. 5,851,947, issued Dec. 22, 1998, discloses a method for incorporating noble metals into inorganic aerogels. The metal precursors were added to the sol. After gelatin, the ethanol was removed by supercritical drying.
Miller et al., J. Electrochem Soc., 144 (No. 12) (1997); Lanngmuir 15:799-806 (1999) disclose the deposition of ruthenium nanoparticles on carbon aerogels. Carbon aerogels were prepared and impregnated with ruthenium 2,4 pentanedionate by chemical vapor impregnation.
Maldonado-Hodar et al., Carbon 37, 1199-1205 (1999), disclose a series of carbon aerogels containing Pt, Pd and Ag. Pt(NH3)4Cl2, PdCl2 and Ag(CH3COO) were used as the polymerization catalyst in the initial solution for preparation of RF aerogels. After curing, water was exchanged with acetone and acetone was extracted by supercritical carbon dioxide. Subsequently, the aerogels were pyrolyzed in flowing nitrogen.
U.S. Pat. No. 5,789,027, issued Aug. 4, 1998, discloses methods for depositing a film of material on the surface of a substrate by i) dissolving a precursor of the material into a supercritical or near-supercritical solvent to form a supercritical or near-supercritical solution; ii) exposing the substrate to the solution, under conditions at which the precursor is stable in the solution; and iii) mixing a reaction reagent into the solution under conditions that initiate a chemical reaction involving the precursor, thereby depositing the material onto the solid substrate, while maintaining supercritical or near-supercritical conditions. The patent also discloses similar methods for depositing material particles into porous solids, and films of materials on substrates or porous solids having material particles deposited in them.
Processes such as described above often have inadequate control over the manner in which the metallic particles are incorporated, thereby providing aerogel compositions having inconsistent metal particle sizes and broad particle size distributions. This has been one of the factors which have inhibited the commercialization of aerogels, particularly for use in PEM fuel cells which currently require large amount of platinum to obtain an acceptable level of performance. Decreasing the amount of platinum used in fuel cells would be beneficial for fuel cell based power generation systems to compete with internal combustion engines.
Accordingly, aerogel compositions comprising aerogels having metallic particles, e.g., platinum, dispersed within, and processes for making such aerogels, are desired. Desirably, such aerogel compositions would contain metal particles having a small particle size, e.g., 4 nanometers or less, with a narrow particle size distribution.